electromagnetism Flashcards
names of the two ends of a magnet
north pole
South Pole
conditions required for attraction between two magnets
opposite poles, near each other
conditions required for attraction between a magnet and a magnetic material
opposite poles, near each other, uniform magnetic field
what type of force is magnetism
non contact force
conditions required for repulsion
same poles
describe a magnetically soft material
something easy to magnetise and demagnetise
describe a magnetically hard material
something difficult to magnetise and demagnetise
name 3 magnetic elements
iron, cobalt, nickel
state a material that would be suitable for making a magnet out of
steel
what is a magnetic field
the area around a magnet that applies a force to magnetic materials
what direction do magnetic fields point
north to south
what happens to a magnetic material when placed in a magnetic field
It would become magnetised, with unlike pole facing the magnet, and is attracted to the magnet
describe how to show the shape and direction of the magnetic field between two like poles
- Place a plotting compass in the field
- Mark the direction
- Move it and mark the direction
- Repeat and join up the dots to show the magnetic field lines
- Repeat multiple times to show the whole field
describe how to show the shape and direction of the magnetic field between two unlike poles
- Place a plotting compass in the field
- Mark the direction
- Move it and mark the direction
- Repeat and join up the dots to show the magnetic field lines
- Repeat multiple times to show the whole field
describe how to show the shape and direction of the magnetic field around a bar magnet
- Place a plotting compass in the field
- Mark the direction
- Move it and mark the direction
- Repeat and join up the dots to show the magnetic field lines
- Repeat multiple times to show the whole field
describe how to use two permanent magnets to produce a uniform magnetic material
opposite poles of 2 magnets near each other
describe a uniform magnetic material
equally spaced
what happens when an electric current flows through a long straight wire?
a magnetic field is produced which rotates in circles around the wire
right hand grip:
point the thumb of your right hand in the direction of the current, and the direction your fingers curl up is the direction of the field
if a vector is pointing towards you what does it look like
a circle with a dot in the middle
if a vector is pointing away from you what does it look like?
X (a cross)
where is the magnetic field strongest and how do you know?
closest to the wire because the field lines are most dense
the larger the current, the ….
stronger the field
the closer you go to the wire, the …
stronger the field will be
how could you reverse a magnetic field around a long, straight wire?
reverse the direction of current flowing through the wire
how could you increase the strength of a magnetic field around a long, straight wire?
increase the current
describe an investigation to find the shape and direction of a magnetic field around a long straight wire
- place a wire through a board/piece of paper
- run a current through the wire
- place a plotting compass around the wire, and mark the direction it points
- repeat this step, until all plotting compass lines connect
how are most electromagnets made?
by passing a current through a coil of wire, called a solenoid
what do the fields do outside the solenoid?
the fields cancel out to make a shape similar to a bar magnet
what do the fields do inside the solenoid?
add up to make a strong uniform magnetic field
how to make an electromagnet:
- wrap a coil of wire around an iron core
- pass a current through the wire
describe how a student could investigate how the number of turns in a solenoid affects the force of attractions of the solenoid.
- wrap a coil of wire around an iron core
- keep the current the same using an ammeter
- measure the number of paperclip attracted to the solenoid
- repeat with varying numbers of turns in the coil
describe how a student could investigate how the size of the current in the solenoid affects the force of attraction between the solenoid and iron nail.
- note the position of the iron nail before the current is switched on
- measure the position of the pointer when the current is in coil
- use an ammeter to measure current
- calculate the stretch of the spring
- force of attraction is proportional to stretch of spring
- repeat with varying currents
how does an electromagnetic bell work?
- current flows through the coil and becomes magnetic
- attracts the armature
- when the armature is attracted to the magnet it breaks the circuit
- the spring pulls the armature back and connects the circuit
explain the function of a circuit breaker
- when too much current flows through a wire the circuit breaker stops the flow of electricity
- this breaks the circuit
- this prevents a fire
how does a circuit breaker work
- a current flows into the electromagnet
- the electromagnet attracts the iron rocker
- when the current is high enough the force of attraction is greater than the spring force and the rocker moves
- the movement of the armature breaks the circuit
what does a moving charged particle produce
a circular magnetic field
what happens if a circular magnetic field is placed perpendicular in a uniform magnetic field of a permanent magnet?
the 2 fields interact, putting a sideways force on the wire and the magnet
how can you find the force, magnetic field or current?
Flemming’s left hand rule
Flemming’s left hand rule:
F.B.I
F = force (thumb finger)
B = magnetic field (index finger)
I = current (middle finger)
current carrying wire in a magnetic field experience a …..
force
what happens if you increase the magnitude of current in a current carrying wire?
increase magnetic field strength of wire, increase magnitude of force on wire
what happens if you reverse the direction of current in a current carrying wire?
reverse direction field of wire, reverse direction of force on wire
motors:
- a wire carrying a coil produces a circular magnetic field
- if this field is placed in the uniform magnetic field of a permanent magnet, the two fields interact
- there is a force on the opposite sides of the coil in opposite directions
- the coil starts to rotate
how could a motor be made more powerful?
- increase current flowing through wire
- this increases the magnetic field strength
- this increases the number of wires in coil
- this increases the area of the coil (this increases the turning effect or momentum)
how do speakers work?
- current in coil produces magnetic field
- field of coil interacts with field of magnet, producing a force on cone
- direction of current is constantly changing because current is a.c
- direction of magnetic field is continuously changing
- direction of the force changes
- cone vibrates
- air particles next to the cone vibrate
what does an electric current in a conductor do
produces a magnetic field around it
why is a force exerted on a current carrying wire in a magnetic field
- the two magnetic fields interact
- results in a force on the wire
- can cause the wire to move
how does the force on a current carrying conductor in a magnetic field change with the magnitude and direction of the field and current
- if the direction of movement is reversed, then the induced current/voltage will be reverse too
how to increase voltage
- increase the strength of the magnet
- increase the number of turns on the coil
- increase the speed of movement
what is electromagnetic induction
the creation of a voltage in a wire which is experiencing a change in magnetic field
the dynamo effect - move the wire or the magnet:
1) using ELECTROMAGNETIC INDUCTION to generate electricity using energy from kinetic energy stores is called the DYNAMO EFFECT
2) there are two different situations where you get EM induction:
- an ELECTRICAL CONDUCTOR (a coil or wire is often used) moves through a MAGNETIC FIELD
- the MAGNETIC FIELD through an electrical conductor CHANGES
3) you can test this by connecting an ammeter to a conductor and moving the conductor through a magnetic field
-> the ammeter will show the MAGNITUDE and DIRECTION of the induced current
4) if the DIRECTION of the movement is REVERSED, then the induced VOLTAGE will be reversed too
how does a simple generator work
1) generators ROTATE A COIL in a MAGNETIC FIELD
2) as the COIL SPINS, a CURRENT is INDUCED in the coil -> this current CHANGES DIRECTION every half turn
3) when the current CHANGES DIRECTION it creates an ALTERNATING MAGNETIC FIELD
4) the coil CUTS through the magnetic field
5) this INDUCES a VOLTAGE
what does a transformer do
changes the size of an alternating voltage
structure of a transformer
have a primary and secondary coil, joined with an iron core
step-up transformers:
- increase the voltage
- they have more turns on the secondary coil than the primary coil
step down transformers:
- decrease the voltage
- they have more turns on the primary coil than the secondary
relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer
input (primary) voltage / output (secondary) voltage = primary turns / secondary turns
what can you rewrite input power = output power as
VpIp = VsIs
V = primary voltage
I = primary current
V = secondary voltage
I = secondary current
step-up and step-down transformers are used when transmitting electricity across the country:
- the voltage produced by power stations is too low to be transmitted efficiently
-> (power = voltage x current) so the lower the voltage, the higher the current for a given amount of power and current causes wires to heat up - a step-up transformer is used to boost the voltage before it is transmitted
- step-down transformers are used at the end of the journey to reduce the voltage so its more useful and safer to use
describe the construction of an electromagnet
run a CURRENT though a COIL of wire, wrapped around an IRON CORE
how does a coil in a magnetic field produce a voltage / how does an electric motor work
- forces acting up and down on the coil makes the coil rotate
- this means the coil cuts the magnetic field
- this induces a voltage
